Optical sensor path selection

ABSTRACT

A device includes a sensor for measuring a parameter for tissue. The sensor includes a plurality of optical elements including a plurality of detectors and at least one emitter. Separation distances between the various optical elements are selected based on a depth corresponding to a region of interest in the tissue and based on a depth corresponding to an exclusion region in the tissue.

CLAIM OF PRIORITY

This patent application is a continuation of and claims the benefit ofpriority, under 35 U.S.C. §120, to Isaacson, U.S. patent applicationSer. No. 14/244,256, entitled “OPTICAL SENSOR PATH SELECTION,” filed onApr. 3, 2014, (Attorney Docket No. 2898.020US2), which is a divisionalof and claims the benefit of priority, under 35 U.S.C. §120, toIsaacson, U.S. patent application Ser. No. 12/618,120, entitled “OPTICALSENSOR PATH SELECTION,” filed on Nov. 13, 2009, (Attorney Docket No.2898.020US1), which claims the benefit of priority, under 35 U.S.C.§119(e), to Isaacson, U.S. Provisional Patent Application Ser. No.61/114,528, entitled “OPTICAL SENSOR PATH SELECTION,” filed on Nov. 14,2008, (Attorney Docket No. 2898.020PRV), and each of which areincorporated herein by reference in their entireties.

BACKGROUND

The human brain requires a continuous supply of oxygen. A measure ofblood oxygenation can help to accurately diagnose a medical condition ormonitor the health of a patient. Current technology for determiningcerebral oximetry is inadequate.

SUMMARY

The present subject matter includes systems and methods as describedherein. For example, a patient sensor includes a first emitter and afirst detector separated by a first dimension and a second emitter and asecond detector separated by a second dimension. The first dimension andthe second dimension can be determined by a particular technique.

In one example, the sensor is fully compensated and include two emittersand two detectors. In this example, a first emitter and a first detectorare coupled by a short path that traverses a surface layer of the tissueas well as an exclusion region within the tissue. The first emitter isalso coupled to a second detector by a long path that traverses thesurface layers of the tissue as well as a region of interest at aparticular depth within the tissue. A second emitter is coupled to thefirst detector by a long path that traverses the surface layers of thetissue as well as the region of interest within the tissue. The secondemitter is also coupled to the second detector by a short path thattraverses the surface layers of the tissue and passes through exclusionregion of the tissue without encroaching on the region of interest.

The mean depth of the light path is approximately one third of thedistance between the emitter and the detector. According to one example,a method includes selecting a long path dimension and selecting a shortpath dimension for placement of detectors and emitters.

Consider first, selecting a long path dimension for a sensor having twoemitters and two detectors. The long path dimension refers to thelateral separation between an emitter and a detector in which the paththrough the biological tissue traverses the region of interest. The longpath dimension is proportional to the average depth of the region ofinterest. In one example, the region of interest is the cerebral cortexand the long path dimension is approximately 40 mm.

Next, consider selecting the short path dimension. The short pathdimension also refers to the separation between an emitter and adetector. The short path dimension is selected to provide an opticalpath having a tissue depth that traverses a surface layer and does nottraverse the region of interest. As with the long path dimension, theshort path dimension is proportional to the penetration depth in thetissue. The optical path corresponding to the short path dimension isselected to be approximately three times the thickness of the surfacelayer to be excluded (e.g., the dermis and epidermis) and just short ofthe depth of the region of interest. A typical scalp thickness isapproximately at least 3 mm and a typical skull thickness isapproximately at least 5 mm which means that the minimum depth to thebrain is approximately 8 mm. Thus, for cerebral oximetry the short pathdimension is selected to be less than three times 8 mm (24 mm). In oneexample, the short path dimension is 20 mm.

More generally, the scalp depth is between approximately 3 mm and 10 mmand the skull depth is between approximately 5 mm and 10 mm.

For a neonate, typical dimensions are 3 mm for the scalp and 4 mm forthe skull. As such, the long path dimension is at least approximatelythree times 7 mm (21 mm). In one example, the long path dimension is 25mm. The short path dimension is at least three times the scalp thickness(9 mm) and less than 21 mm. In one example, the short path dimension is12.5 mm.

In one example, the long path dimension is twice that of the short pathdimension. For example, an adult cerebral oximetry sensor has a longpath dimension and short path dimension of 25 mm and 12.5 mm,respectively and a neonate cerebral oximetry sensor has dimensions of 40mm and 20 mm, respectively. The 2:1 ratio between long dimension andshort dimension provides good compensation and good signal; however,other ratios are also contemplated.

This summary is intended to provide an overview of subject matter of thepresent patent application. It is not intended to provide an exclusiveor exhaustive explanation of the invention. The detailed description isincluded to provide further information about the present subjectmatter.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, which are not necessarily drawn to scale, like numeralsmay describe similar components in different views. Like numerals havingdifferent letter suffixes may represent different instances of similarcomponents. The drawings illustrate generally, by way of example, butnot by way of limitation, various embodiments discussed in the presentdocument.

FIG. 1 includes a view of a sensor according to one example.

FIG. 2 includes a system according to one example.

FIG. 3 includes a method according to one example.

FIG. 4 includes a method according to one example.

DETAILED DESCRIPTION

The present subject matter is directed to in vivo optical examinationand monitoring of selected blood metabolites or constituents in human orother living subjects. Examination and monitoring can includetransmitting selected wavelengths of light into a particular area ofbiological tissue and receiving the resulting light as it emerges fromthe area, and analyzing the received light to determine the desired databased on light absorption.

One example includes an optical sensor assembly that is particularlyadapted for in vivo use as the patient interface in a patient-monitoringapparatus such as a cerebral or tissue oximeter.

One example can be used for non-invasive determination of tissueoxygenation or non-invasive cerebral oximetry. Cerebral oximetryprovides a measure of blood oxygen saturation in the brain. One exampleincludes an optical sensor having light emitters and detectors that canbe applied to the forehead of the patient.

One example includes an apparatus for in vivo monitoring of bloodmetabolites such as hemoglobin oxygen concentration in any of aplurality of different regions of a patient through application of anoptical sensor assembly. The optical sensor assembly is in communicationwith, or is coupled to, a processor.

The processor can be configured to control the sensor and analyze datafrom the sensor. One example of a processor includes a monitor whichprovides a visible display based on the analysis.

The processor can be configured to operate the sensor. The sensor isconfigured to couple with tissue of the patient and emit and detectlight energy. The sensor provides an output signal to the processorcorresponding to the detected energy.

One example includes an optical probe configured to conform to a shapeof the cerebrum or other anatomical area.

FIG. 1 illustrates view 100 including processor 30A, sensor 34A (inpartial sectional view), and biological tissue 50 (also in partialsectional view) according to one example.

Processor 30A is in communication with, or is coupled to, sensor 34A bylink 32A. Processor 30A can include a digital processor, a centralprocessor unit (CPU), a microprocessor, a computer, a digital signalprocessor, an application specific integrated circuit (ASIC), an analogprocessor, or a mixed signal processor.

In addition, processor 30A can include a memory or other device forstoring instructions or data. Processor 30A can include other elementsas well, including, for example, an analog-to-digital converter (ADC), adigital-to-analog converter (DAC), a driver, an amplifier, a filter, orother circuitry to perform a method as described herein.

Link 32A can include a wired or wireless channel. Link 32A can convey aninstruction or control signal. Link 32A can convey a communicationsignal or data corresponding to a detected signal.

Sensor 34A includes housing 38 having a surface 36. Housing 38 can berigid or flexible and is configured for coupling to biological tissue 50at surface 36. In the example shown, surface 36 is closely conformed tothe contours of biological tissue 50. Sensor 34A can be affixed tobiological tissue by adhesive, a strap, a band, a clamp, or other means.

Sensor 34A includes first emitter 10, second emitter 20, first detector12, and second detector 22. Emitters 10 and 20 and detectors 12 and 22are positioned about surface 36 in a manner that allows optical signalsto freely pass between sensor 34A and biological tissue 50. In oneexample, emitters 10 and 20 and detectors 12 and 22 are mounted to anelectrical circuit (such as a printed wire board, a substrate, rigidcircuit board, or flexible circuit material) within sensor 34A andoptical energy passes through an aperture or window in surface 36.

In one example, at least one of first emitter 10 and second emitter 20includes a light emitting diode (LED). In the figure, first emitter 10and second emitter 20 are shown as unitary devices but in variousexamples, either can include multiple individual LEDs configured toproduce light of a particular wavelength. In one example, first emitter10 and second emitter 20 include a fiber-optic element. The energyemitted by emitter 10 or emitter 20 can include visible light, infraredenergy, and near infrared energy. In one example, first emitter 10produces light of a particular wavelength and second emitter 20 produceslight of a different wavelength. First emitter 10 and second emitter 20are coupled to processor 30A by link 15 and link 25, respectively.

In one example, at least one of first detector 12 and second detector 22includes a photodetector. First detector 12 and second detector 22 areconfigured to generate an output based on received energy having aparticular wavelength. The sensitivities of first detector 12 and seconddetector 22 can be selected (or adjusted) to generate an output forparticular wavelengths. First detector 12 and second detector 22 arecoupled to processor 30A by link 19 and link 29, respectively.

In addition to sensor 34A, FIG. 1 illustrates biological tissue 50.Biological tissue 50, in the example shown, depicts a portion of a humanforehead; however other biological tissue can be monitored as well. Forexample, the present subject matter can be used with an arm, a finger(or thumb), a toe, an ear lobe, and a torso.

Biological tissue 50, as illustrated, includes a plurality of layers. Asshown in the figure, the layers include scalp 52, skull 54, dura 56,arachnoid 58, pia mater 60, and cerebral cortex 62. In the figure, eachlayer has a relatively uniform thickness however; this can vary fromsite to site of a particular patient as well as from one patient to thenext. A typical thickness for scalp 52 is in the range of 3 mm to 10 mmand for skull 54, the typical thickness is between 5 mm and 10 mm. Assuch, the brain (cerebral cortex) is typically at a depth of greaterthan 8 mm below the exterior surface of scalp 52.

FIG. 1 illustrates region of interest 18 and exclusion region 28. In theexample shown, region of interest 18 lies wholly within the layer ofcerebral cortex 62 at an average depth denoted by first depth 17. Regionof interest 18 is representative of a portion of the cerebral cortex.Exclusion region 28 extends from the surface of biological tissue 50 tonearly the region of interest 18 and has an average depth denoted bysecond depth 27. Exclusion region 28, in the example shown includesscalp 52, skull 54, dura 56, arachnoid 58, pia mater 60, and a portionof cerebral cortex 62.

In other examples, region of interest 18 and exclusion region 28 mayoccur in layers other than that shown in the figure. For example, regionof interest 18 can lie in cerebral cortex 62 and exclusion region 28 caninclude the layers of dura 56, arachnoid 58, and pia mater 60. In oneexample, region of interest 18 can lie in a first portion of cerebralcortex 62 and exclusion region 28 can include a second portion ofcerebral cortex 62 where the first portion has a depth of 10 mm and thesecond portion has a depth of 8 mm. The depth of exclusion region 28 isless than the depth of the region of interest 18.

As shown in the figure, energy emitted from first emitter 10 can bemodeled by path 16A and by path 26A. Path 16A enters biological tissue50, traverses region of interest 18, and emerges from biological tissue50 and the resulting energy is detected by second detector 22. Path 26Aenters biological tissue 50, traverses exclusion region 28, and emergesfrom biological tissue 50 and the resulting energy is detected by firstdetector 12. In a similar manner, energy emitted from second emitter 20can be modeled by path 16B and by path 26B. Path 16B enters biologicaltissue 50, traverses region of interest 18, and emerges from biologicaltissue 50 and the resulting energy is detected by first detector 12.Path 26B enters biological tissue 50, traverses exclusion region 28, andemerges from biological tissue 50 and the resulting energy is detectedby second detector 22.

To the extent that paths 16A and 16B and paths 26A and 26B are models,the actual path followed by energy delivered by sensor 34A may bedifferent than that shown. For example, light scattering and otheroptical effects can change the actual path through biological tissue 50.Paths 16A, 16B, 26A, and 26B represent a mean path by which lighttraverses biological tissue 50. In general, the light traverses thetissue in a curved shape that resembles a banana.

Path 16B and path 26A illustrate that energy detected by first detector12 originates from second emitter 20 and first emitter 10, respectively.In a similar manner, path 26B and path 16A illustrate that energydetected by second detector 22 originates from second emitter 20 andfirst emitter 10, respectively.

First emitter 10 is separated from first detector 12 by a lateraldistance denoted in the figure as dimension 24A and is separated fromsecond detector 22 by a lateral distance denoted in the figure asdimension 14A. In a similar manner, second emitter 20 is separated fromsecond detector 22 by a lateral distance denoted in the figure asdimension 24B and is separated from first detector 12 by a lateraldistance denoted in the figure as dimension 14B. Dimension 14A anddimension 14B are approximately equal and dimension 24A and dimension24B are approximately equal. Dimension 14A (and thus dimension 14B) isapproximately twice the length of dimension 24A (and thus dimension24B), thus having a ratio of approximately 2:1.

The depth of energy penetration into biological tissue 50, and thus thedepth of the region (region of interest 18 or exclusion region 28) areproportional to the corresponding lateral distance. To a closeapproximation, the depth of penetration is approximately one third thelateral distance at the surface of biological tissue 50.

FIG. 2 illustrates system 200 according to one example. System 200includes sensor 34B coupled by link 32B to a module, here shown toinclude processor 30B. Sensor 34B is affixed to a forehead of biologicaltissue 50 (depicted herein as that of an infant or neonate), however,sensor 34B can be affixed to another particular site of a human. Sensor34B includes a pair of emitters and a pair of detectors as describedelsewhere in this document, and in the example shown, is depicted asadhesively coupled to tissue 50. Link 32B is illustrated as a wiredconnection however, a wireless coupling is also contemplated. Forexample, link 32B can include an optical fiber or a short-range radiofrequency (RF) transceiver.

Processor 30B is shown coupled to output 220. Output 220 can include, invarious examples, a visual display, a memory, a printer, a network (dataor communication), a speaker, or other such device. In one example,processor 30B generates a processor output that is communicated tooutput 220. In one example, processor 30B and output 220 are part of astand-alone unit typically referred to as monitor 210. Monitor 210 canbe configured for patient use or for use by medical personnel.

FIG. 3 includes method 300 according to one example. At 305, method 300includes identifying a region of interest. The region of interest caninclude the cerebral cortex, a muscle, or other substance at aparticular depth within biological tissue. At 310, method 300 includesselecting a path depth based on the region of interest. The pathtraverses the biological tissue and the region of interest at aparticular depth. In the example shown in FIG. 1, a representative pathdepth is depicted as first depth 17.

The path can be projected onto an adjacent surface of the biologicaltissue to yield a spacing dimension. At 315, method 300 includesestablishing the dimension between the emitter and the detector. Asshown in the example of FIG. 1, this corresponds to, for example,dimension 14A. For some biological tissue, the path length and depth arerelated by ratio of 3:1.

Method 300 represents a general procedure for selection of a pathlength. The discussion has focused on the region of interest but asimilar calculation can be performed for the region denoted earlier asthe exclusion region.

FIG. 4 includes method 400 according to one example. At 405, method 400includes identifying a region of interest disposed in a first layer of abiological tissue. With respect to the example of FIG. 1, region ofinterest 18 lies in the layer of cerebral cortex 62 of biological tissue50. At 410, method 400 includes identifying an exclusion region disposedin a second layer of the biological tissue. FIG. 1 illustrates exclusionregion 28 within the layer of scalp 52, skull 54, dura 56, arachnoid 58,pia mater 60, and also cerebral cortex 62. As shown, exclusive region 28occupies a different layer than that of region of interest 18. Inparticular, the regions are exclusive of each other. In addition, thedepth of region of interest 18 (depth 17) is greater than that of thedepth of exclusion region 28 (depth 27).

At 415, method 400 includes selecting a value for depth 17 correspondingto region of interest 18, and at 420, selecting a value for depth 27corresponding to the exclusion region 28. The second depth is less thanthe first depth, and in one example, the second depth is less than 80percent of the first depth. For example, with an 8 mm value for firstdepth 17, the value for second depth 27 is 6.4 mm.

At 425, method 400 includes using depth 17 to determine dimension 14A(between first emitter 10 and second detector 22) and to determinedimension 14B (between second emitter 20 and first detector 12).

At 430, method 400 includes using depth 27 to determine dimension 24A(between first emitter 10 and first detector 12) and to determinedimension 24B (between second emitter 20 and second detector 22).

With reference to FIG. 1, first detector 12 generates a first outputbased on the optical coupling with first emitter 10 (via exclusionregion 28) and second emitter 20 (via region of interest 18). The firstoutput from first detector 12 can include an analog or digital signalprovided by a photodetector. In similar manner, second detector 22generates a second output based on the optical coupling with secondemitter 20 (via exclusion region 28) and first emitter 10 (via region ofinterest 18).

Processor 30A uses the first output (from first detector 12) and thesecond output (from detector 22) to determine a parameter for thebiological tissue. The parameter, for example can include a measure ofblood oximetry or tissue oximetry.

The first output and the second output can be configured to selectivelycorrespond to the region of interest 18 or the exclusion region 28. Forexample, an emitter (such as emitter 10 or emitter 20) can be configuredto produce a particular wavelength of light. In addition, a detector(such as detector 12 or detector 22) can be configured for sensitivityto light having a particular wavelength.

In one example, the emitters and the detectors are sequentiallyactivated. For example, the emitters are sequentially powered and thenun-powered in order to generate data corresponding to the different pathlengths. Other techniques and arrangements to encode the data producedby the various emitter-detector pairs are also contemplated.

With reference to both FIG. 4 and FIG. 1, at 435, method 400 includesusing a calculated dimension 14A to position first emitter 10 relativeto second detector 22 in housing 38 of sensor 34A and usingapproximately the same dimension 14B to position second emitter 20relative to first detector 12. In one example, this includes affixingfirst emitter 10 and first detector 12 at a spacing of 40 mm.

At 440, method 400 includes using a calculated dimension 24A to positionfirst emitter 10 relative to first detector 12 in housing 38 and usingapproximately the same dimension 24B to position second emitter 20relative to second detector 22 in housing 38. In one example, thisincludes affixing second emitter 20 and second detector 22 at a spacingof 20 mm. Sensor 34A is configured to determine a physiologicalparameter of biological tissue 50.

In one example, processor 30A executes instructions to determineoxygenation or other physiological parameter using the first output(from first detector 10) and the second output (from second detector20). This can include executing an instruction to perform an algorithmwherein the instructions are stored in a memory accessible to processor30A. In one example, the instructions can include using a look-up tablestored in a memory.

In one example, first dimension 14A (and dimension 14B) and seconddimension 24A (and dimension 24B) are related by a ratio of 2:1. Inother words, the value of dimension 14A is twice that of dimension 24A.For example, first dimension 14A and second dimension 24A can be 40 mmand 20 mm, respectively, or 25 mm and 12.5 mm, respectively.

Additional Examples

The arrangement of optical elements (emitters and detectors) in FIG. 1can be modeled as emitter-detector-detector-emitter and the dimensionsbetween the various elements can be determined as described elsewhere inthis document. For a sensor having four elements a variety ofarrangements having a different layout, different spacing, or adifferent order of elements are contemplated. For example, the elementscan be arranged as detector-emitter-emitter-detector, asdetector-detector-emitter-emitter, or asdetector-emitter-detector-emitter.

A sensor can have three elements including, for example, two detectorsand a single emitter. As with the other configurations described herein,the short dimension is selected to produce a shallow path through anexclusion region of the tissue and the long dimension is selected toproduce a deep path through a region of interest in the tissue.

In addition, a sensor having more than four elements is alsocontemplated. The elements can be arranged in an array of emitters and acorresponding array of detectors. Either or both array can be of asingle dimension (e.g., an array of four emitters) or of two dimensions(e.g., an array of three detectors by four detectors).

In one example, a first housing of the sensor can include one or moreemitters and a second housing of the sensor can include one or moredetectors. The first housing and the second housing are coupled by awired or wireless link and the placement of the housings is userselectable.

In one example, a sensor includes a number of detectors that differsfrom a number of emitters. For example, a particular sensor can have asingle emitter and two detectors wherein the path through the region ofinterest differs from the path through an exclusion region, and thus,the first dimension differs from second dimension as described elsewherein this document. In one example, more than two emitters and more thantwo detectors are included in a particular sensor.

In one example, a sensor includes an array of emitters in which thearray is configured with individual elements that can selectively emitenergy in order to provide a variable dimension. In a similar manner,one example includes an array of photodetector elements, also configuredfor individual selection to provide a variable dimension.

In one example, an emitter produces a white light and a particulardetector is configured for sensitivity for a particular wavelength oflight corresponding to the parameter being measured.

In one example processor 30A calculates a physiological parameter usingan algorithm in which the optical absorbance corresponding to the shortpaths is subtracted from the optical absorbance corresponding to thelong paths.

-   -   Additional Notes

The above detailed description includes references to the accompanyingdrawings, which form a part of the detailed description. The drawingsshow, by way of illustration, specific embodiments in which theinvention can be practiced. These embodiments are also referred toherein as “examples.” Such examples can include elements in addition tothose shown and described. However, the present inventors alsocontemplate examples in which only those elements shown and describedare provided.

All publications, patents, and patent documents referred to in thisdocument are incorporated by reference herein in their entirety, asthough individually incorporated by reference. In the event ofinconsistent usages between this document and those documents soincorporated by reference, the usage in the incorporated reference(s)should be considered supplementary to that of this document; forirreconcilable inconsistencies, the usage in this document controls.

In this document, the terms “a” or “an” are used, as is common in patentdocuments, to include one or more than one, independent of any otherinstances or usages of “at least one” or “one or more.” In thisdocument, the term “or” is used to refer to a nonexclusive or, such that“A or B” includes “A but not B,” “B but not A,” and “A and B,” unlessotherwise indicated. In the appended claims, the terms “including” and“in which” are used as the plain-English equivalents of the respectiveterms “comprising” and “wherein.” Also, in the following claims, theterms “including” and “comprising” are open-ended, that is, a system,device, article, or process that includes elements in addition to thoselisted after such a term in a claim are still deemed to fall within thescope of that claim. Moreover, in the following claims, the terms“first,” “second,” and “third,” etc. are used merely as labels, and arenot intended to impose numerical requirements on their objects.

Method examples described herein can be machine or computer-implementedat least in part. Some examples can include a computer-readable mediumor machine-readable medium encoded with instructions operable toconfigure an electronic device to perform methods as described in theabove examples. An implementation of such methods can include code, suchas microcode, assembly language code, a higher-level language code, orthe like. Such code can include computer readable instructions forperforming various methods. The code may form portions of computerprogram products. Further, the code may be tangibly stored on one ormore volatile or non-volatile computer-readable media during executionor at other times. These computer-readable media may include, but arenot limited to, hard disks, removable magnetic disks, removable opticaldisks (e.g., compact disks and digital video disks), magnetic cassettes,memory cards or sticks, random access memories (RAMs), read onlymemories (ROMs), and the like.

The above description is intended to be illustrative, and notrestrictive. For example, the above-described examples (or one or moreaspects thereof) may be used in combination with each other. Otherembodiments can be used, such as by one of ordinary skill in the artupon reviewing the above description. The Abstract is provided to complywith 37 C.F.R. §1.72(b), to allow the reader to quickly ascertain thenature of the technical disclosure. It is submitted with theunderstanding that it will not be used to interpret or limit the scopeor meaning of the claims. Also, in the above Detailed Description,various features may be grouped together to streamline the disclosure.This should not be interpreted as intending that an unclaimed disclosedfeature is essential to any claim. Rather, inventive subject matter maylie in less than all features of a particular disclosed embodiment.Thus, the following claims are hereby incorporated into the DetailedDescription, with each claim standing on its own as a separateembodiment. The scope of the invention should be determined withreference to the appended claims, along with the full scope ofequivalents to which such claims are entitled.

1. (canceled)
 2. A method comprising: identifying a region of interestdisposed in a first layer of a biological tissue; identifying anexclusion region disposed in a second layer of the biological tissue,the first layer exclusive of the second layer and the first layer havinga greater depth than the second layer; selecting a first depthcorresponding to the region of interest; selecting a second depthcorresponding to the exclusion region; using the first depth todetermine a first dimension between a first emitter and a seconddetector and between a second emitter and a first detector, the firstdetector configured to generate a first output and the second detectorconfigured to generate a second output; using the second depth todetermine a second dimension between the first emitter and the firstdetector and between the second emitter and the second detector; usingthe first dimension to position the first emitter relative to the seconddetector in a housing of a sensor and to position the second emitterrelative to the first detector in the housing; and using the seconddimension to position the first emitter relative to the first detectorand to position the second emitter relative to the second detector inthe housing, the sensor configured to determine a physiologicalparameter of the biological tissue.
 3. The method of claim 2 whereinselecting the first dimension includes selecting approximately 40 mm. 4.The method of claim 2 wherein selecting the second dimension includesselecting approximately 20 mm.
 5. The method of claim 2 whereinselecting the first dimension includes selecting approximately 25 mm. 6.The method of claim 2 wherein selecting the second dimension includesselecting approximately 12.5 mm.
 7. The method of claim 2 furtherincluding configuring the housing for attachment to a forehead of apatient.
 8. The method of claim 2 wherein the housing is configured foraffixation proximate the skull.
 9. The method of claim 2 wherein thefirst detector and the second detector are configured to provide ameasurement signal corresponding to cerebral oximetry.
 10. The method ofclaim 2 wherein the first dimension and the second dimension areconfigured to provide a measurement signal corresponding to cerebraloximetry for a neonate.
 11. The method of claim 2 wherein the firstemitter and the second emitter are configured to emit light at aselected wavelength.
 12. The method of claim 2 further includingcoupling a processor to the first detector, the second detector, thefirst emitter, and the second emitter.
 13. The method of claim 12further including configuring the processor to determine tissueoximetry.
 14. The method of claim 12 further including configuring theprocessor to provide an output signal corresponding to metabolites inthe region of interest.
 15. The method of claim 12 further includingconfiguring the processor to sequentially power and un-power the firstemitter and the second emitter.
 16. The method of claim 12 furtherincluding configuring the processor to selectively activate the firstemitter, the second emitter, the first detector, and the seconddetector.
 17. The method of claim 12 further including configuring theprocessor to determine a physiological parameter based on addition andsubtraction of signals from the detectors.
 18. The method of claim 12further including configuring the processor to determine an outputsignal in which optical absorbance corresponding to a short path issubtracted from optical absorbance corresponding to a long path.